Never heard of Krown. What are you rust proofing? I've used fish oil inside bike frames and cars.

Krown is very big in the Canadian car rust-proofing business. The MSDS on the product is pretty non-contributory, but it is a petroleum product with moisture displacers and rust inhibitors. It has to be applied every year. If you live in a snowy area that uses salt to de-ice the roads, it may be a good idea.

We have a rust proofer around here called Ziebart, and they do exceptional work, given that this region is in my experience the most rust ridden in all of the USA. And yes, it's much more than spraying oil. They clean the underside, and apply a thick coat of rubber. Small chunks can get flaked off, but the whole thing does NOT have to be redone every year, not if you're having the good stuff done as opposed to the cheap stuff in a can. They also get inside your rockers and other body work with atomized rubber and other coatings and rust proof the living hell out of it. I am not so familiar with krown, but my understanding is that they do the same sort of work.

Modern rust proofing is nothing short of amazing. If you have a car you want to drive in the winter without trashing it, have it rust proofed, and have them touch it up every year too. It's almost like having your car's underside turned into a boat bottom in how little it ends up mattering if you get it covered in road salt. The first time I saw a car with this done, I laughed really hard because of how ridiculously effective it is. Totally worth the money.

The one thing to understand is that if you have a car that is already a rust bucket, nothing will stick to it for long unless you sand or other media blast all the rust off, so it's best to have done to a clean car.

The stuff Michigander mentioned is the best... My 30 year old car still has the original rubber undercoating, and there isn't a bit of rust except surface rust on stationary suspension parts pretty much. The stuff you rub on a lot of people swear by. If you're trying to rust-proof an already rusty part, something like POR 15 or "Miracle paint" work wonders. You can even use them in conjunction with fiberglass mesh and resin to rebuild a rusted out part.

There is no one "best" rustproofing product. It really comes down to what are you rustproofing?
For autos, specialized products like Michigander mentioned are best. They are a multi-layer process.

Some imported cars come with zinc-galvanized bodies. It's rust-resistant even before the primer and paint is applied. These cars resist rock-chips, sand and salt for quite a long time. If you're in a rust-prone area, you may want to consider one or these cars if you're going to be living there in the next 10-20 years, as the cost of maintenance and rust-fighting will be much lower in the long-run.

If you're dealing with an existing rusted part, it's really, really difficult as it's pretty much impossible to stop rust once it's started. You have to grind/cut off all the metal that has any rust on it because rust alternates between ferric and ferrous-oxide and will creep laterally beneath the paint without needing any additional oxygen. In this case, after grinding/cutting off all the rusted metal, you can then treat the bare surfaces with a conversion-coating. It's a reduction-reaction that attempts to remove the oxygen from any microscopic rust on the surface that's not visible. Then you cover all that up with etching-primer, sanding-primer, then paint. A lot of work, but much better and longer lasting solution than any spray-on product.

A friend of mine got his Bridgestone repainted and the internal tubes rust-proofed by Ziebart. The problem is that it adds weight to a lightweight frame and after fitting all the parts back on the bike...it just doesn't feel the same. The frame feels dead, lifeless. The bike is not responsive anymore. In my opinion, the bike is ruined.

We had a couple of rust spots fixed on our old Subaru and got an oil treatment - lots of oil stains on the driveway but no more rust.

We had a "Sym-tech" electronic module attached to our current Forester - there are differing views out there as to whether they are effective. No rust after 2 years, but now we have to trade in to get an automatic shift. This thread reminds me to ask the dealer to transfer the module to the next car - just in case it works!

Thanks for all the tips. I should have been more specific. I have a 1984 Dodge D-50 with no rust. But bought a second hand door that appears to have been sitting outside a while. Before I install it I want to apply something that will soak right through the folded seams and all the nooks and crannies. My first thought was fishoilene. However, fishoilene will eventually go semi-hard and I would prefer a product that I can reapply on an annual basis so that I can re-soak the joins etc.

I still recommend fishoil. It seeps into everything even travels upwards in seems and cracks while wet and it's a good thing it dry's eventually because dirt will get in it permanently, don't get it on glass it's a bastard to get off.
I bought 4 litres from Auto Pro and did my Alfa with a cheap long nozzle'd sand blaster gun connected to a air compressor with all the interior removed and in the chassis rails. I don't expect it to rust for 20 years at least probably more.
Google it to find out more from people who have used it long term.